Steps to reproduce:
1. Download project from https://oc.unity3d.com/index.php/s/dfSJhM5LgyAIS0S/download?path=%2F&files=urbanempire-hang-reduced-size-1gb.zip (ownCloud)

2. Download save files from http://bugs04.unity3d.com/fogfiles/846096_dji2tprc7no4o4hq/case_846096-Save.zip (Fogbugz)
3. Unzip the save files to:
(WIN) C:\Users\<username>\AppData\LocalLow\REBORN\Urban Empire\Save (make the savefolder if neeeded)
(OSX) /Users/Ricardo/Library/Application Support/REBORN/Urban Empire/Save (make the savefolder if neeeded)
4. Open the project in Unity
5. Open scene "main_menu"
6. Start a new game
7. Wait till the game loads
8. Press "Escape" to open the Menu
9. Select "Load Game"
10. Click "X" in the right top corner

Result: editor hangs, RAM usage keeps building up, eventually the editor crashes

Reproduced on 5.4.2f1, 5.4.2p3

Note: the user notes that "simply by changing the hierarchy so that the load/save dialog pops up couple of parents upwards in hierarchy makes the hang go away", I was not able to reproduce this